Comment by Zambyte
> A side effect of proposal #23367, which is needed for determining upper bound stack size, is guaranteed de-virtualization when there is only one Io implementation being used (also in debug builds!).
> In the less common case when a program instantiates more than one Io implementation, virtual calls done through the Io interface will not be de-virtualized, as that would imply doubling the amount of machine code generated, creating massive code bloat.
From the article
I wonder how massive it actually would be. I'm guessing it really wouldn't be all that massive in practice even if it of course is easy to create massive examples using ways people typically don't write code.